Continuous casting of steel is carried out in such a way that: molten steel is poured from a ladle via a tundish into a mold; and after a solidified shell forms in the mold, a slab including an unsolidified area is withdrawn downward underneath the mold. When a continuous casting machine is operated, especially when molten steel is cast at high speed, there is a case where part of the solidified shell is constrained from being withdrawn by stick on an inner wall of the mold and this constrained part functions as a hindrance to formation of a normal solidified shell. In this case, not only various faults but also breakout might occur in products.
Conventionally, powder to be put into molten steel in a mold is selected to deal with this problem. Molten powder floats and spreads over the surface of the molten steel, is supplied to a space between the mold and the solidified shell, and functions as a lubricant reducing frictional force between them. Whereby, stick of the solidified shell on the inner wall of the mold can be suppressed in some degree.
However, in recent years, operation of continuous casting has been applied for various kinds of steel grades, and carried out under various casting conditions. Therefore, there is a limit if physical properties of powder are changed to deal with such various situations. Thus, such a method is tried that a mold is oscillated at the same time when powder is put. Proper oscillation of the mold makes it possible to suppress stick in the mold.
Patent Literature 1 discloses applying, to a casting mold, vertical oscillation, having a deviated sine waveform that is deviated from a sine waveform. Patent Literature 1 gives the following formula (X) as a specific deviated sine waveform:Z=a1 sin 2πft+a2 sin 4πft+a3 sin 6πft  (X)where Z is displacement of the mold (mm), a1, a2, a3, . . . are amplitude (mm), f is oscillation frequency of the mold (cycles/s) and t is time(s).
According to Patent Literature 1, oscillation having the waveform represented by the above formula (X) is controlled so that:
(i) the maximum descending speed of the mold during negative strip time is fast;
(ii) the maximum ascending speed of the mold during positive strip time is slow;
(iii) the negative strip time is short; and
(iv) the positive strip time is long,
compared to the case where the oscillation waveform is a sine wave.
The negative strip time is time when the descending speed of the mold is faster than the withdrawal rate of an unsolidified slab. The positive strip time is time when the speed of the mold is slower than the withdrawal rate of the unsolidified slab. According to Patent Literature 1, meeting the requirements of the above (i) to (iv) makes it possible to increase the inflow of molten powder into a space between the mold and the solidified shell and to suppress occurrence of breakout.
However, in the method of Patent Literature 1, the movement of the mold suddenly changes from the ascent to the descent upon the oscillation of the mold. At this time, molten powder adhered in the vicinity of meniscus in the mold and unmolten powder are involved in molten steel. Whereby, the surface quality of a slab deteriorates and/or troubles on the operation occur depending on a type of powder used.
Conventionally, an oscillator including an electric motor and an eccentric cam is used for oscillating a mold. A desired oscillation waveform is obtained according to a shape of an eccentric cam. In this case, an eccentric cam corresponding to an oscillation waveform has to be prepared for changing the oscillation waveform. In recent years, an electro-hydraulic oscillator has been used for oscillating a mold, which has made it easy to change parameters when a mold is oscillated with complex waveforms as disclosed in Patent Literature 1 and Patent Literature 2 below.
Patent Literature 2 discloses the method for operating a continuous casting machine comprising vertically vibrating a mold with the waveform expressed by the formula (Y) below:Z=A(sin 2πft+b cos 4πft+c)  (Y)where Z is displacement of the mold (mm), A is ½ of a vibration stroke S of the mold (mm), b is strain constant, c is strain constant, f is vibration frequency of the mold (Hz/60) and t is time(s).
According to Patent Literature 2, employment of such a vibration waveform makes it possible that abrupt change in the mold from an ascent to a descent does not occur, and molten and unmolten powder are not involved in molten steel.
When such a vibration waveform is employed, a neutral position of the oscillation shifts to either upper or lower side. In this case, symmetry of the oscillation is secured in vertical type continuous casting, in which a path where an unsolidified slab travels in a mold is in a perpendicular direction. On the contrary, in curved type continuous casting, in which a path where an unsolidified slab travels in a mold curves, symmetry of oscillation is broken, and such a problem tends to be arose like poor lubrication in the mold and involvement of powder into molten steel.
If the above vibration waveform in Patent Literature 2 is employed, the displacement Z at the time t=0 is not 0 but SC/2. In this case, a mold cannot oscillate with a predetermined oscillation waveform at the start of operation of an oscillator that oscillates the mold, and the mold is displaced step by step as time passes, for example. This disables a dummy bar, which seals an opening in the bottom side of the mold at the start of casting, to seal an opening enough, and molten steel might leak out of the mold.